It is known that metal surfaces consisting of aluminum, steel and galvanized steel may be provided with thin phosphating layers forming a suitable base for coating with an organic paint.
In addition to zinc ions and phosphate ions, the phosphating solutions employed for this purpose may contain other ions including nickel, manganese, magnesium, calcium, copper, cobalt, alkali and/or ammonium. The phosphating solutions can also contain accelerating additives, such as nitrate, nitrite, chlorate, bromate, peroxide, m-nitrobenzene sulfonate, nitrophenol or combinations thereof. They may also contain anions, such as chloride, nitrate, and sulfate, for preserving electrical neutrality, and grain-refining additives, such as hydroxy-carboxylic acids, aminocarboxylic acids, or condensed phosphates.
Additionally, complex and/or simple fluorides can be present for the treatment particularly of aluminum and its alloys.
A problem which often arises in the phosphating of metal surfaces that consist entirely or in part of aluminum is that grey, uniformly closed zinc phosphate layers are not formed on the surface or surface portions of aluminum. Rather, coatings are formed which sometimes can even be wiped off. These coatings have unsatisfactory adherence and an unsatisfactory resistance to corrosion when a paint is subsequently applied.